demux_lavf: blacklist "tty" libavformat demuxer

This is completely useless, and in this particular case, it broke the
fallback for MLP2 subtitles (stored as .txt files) to demux_subreader.
(Yes, libavformat should be fixed to handle this, but for now this will
_always_ break playback of subtitle files stored in .txt.)

You can still force this demuxer, but by default we will just pretend
that the "tty" demuxer does not exist.